The present invention relates to completing multiple production zones from a single wellbore and in one aspect relates to a whipstock which is used to drill a lateral well into an upper productive zone from a wellbore wherein the whipstock has openings therethrough which, in turn, can be opened after the lateral has been completed to allow flow from lower zones through the whipstock.
Hydrocarbons (i.e. oil and gas) have been routinely produced by drilling and casing a single, main wellbore (e.g. a substantially vertical wellbore) downward from the surface into a lower, primary production subterranean formation or a zone within the formation. In doing so, the wellbore often passes through other lesser-productive formation(s) or zones (s) which lie above the primary production formation. By casing the wellbore substantially throughout its depth, production from these upper formations is initially blocked by the well casing. Due to the low productivity expected from these upper formation(s), it is usually impractical from a commercial standpoint to merely perforate the casing adjacent these formations and commingle this production with that from the lower primary productive formation.
Recently, due to conservation and other considerations, it is becoming more desirable to recover hydrocarbons from these lesser-productive formation(s) or zone(s), especially as the production from the primary formation begins to decline. One known technique for doing this involves drilling one or more xe2x80x9clateralsxe2x80x9d or xe2x80x9cdrain-holesxe2x80x9d substantially horizontally outward into the formation from the main wellbore. As understood in the art, these laterals significantly increase the drainage area around the main wellbore and provide an unrestricted flowpath for fluids from the outer regions of the formation directly into the main wellbore.
Typically, a lateral is drilled by first setting a xe2x80x9cwhipstockxe2x80x9d or diverter in the well casing at a point adjacent the upper production formation. A work string having a mill on the lower end thereof is lowered and deflected off the whipstock to mill a window in the well casing adjacent the formation. The mill is then replaced with a drill bit and the lateral is drilled out into the formation through the window in the casing.
In the prior art, the whipstock is typically landed onto a packer which, in turn, blocks flow through the wellbore at that point. That is, production from the primary, lower formation can no longer flow through the wellbore to the surface once the whipstock assembly is in place. This may seriously affect the economics of the well since the lower, primary formation may still contain a significant amount of recoverable hydrocarbons. Accordingly, it is desirable to maintain fluid communication with the primary formation after a lateral has been completed into an upper formation so that both formations can be produced to the surface.
One way to accomplish this is to use a whipstock which is retrievable through the well casing once the lateral has been completed. However, this can be extremely difficult, if possible at all, to actually carry out commercially in the most wells. Further, it has been proposed that the face of the whipstock be xe2x80x9cperforatedxe2x80x9d with a special perforating gun after the lateral has been completed. This is to provide openings through the whipstock which, in turn, allows the production from the lower formation to flow through the whipstock and on to the surface. However, since the target area or face of a typical whipstock is long but quite narrow, the proper positioning of the perforating gun in relation to the face of the whipstock would be extremely difficult, if possible at all, in most field applications. Accordingly, it is highly likely that the orientation of the gun would be such that no perforations would be formed in the face of the whipstock and therefore no flow through the whipstock would be established.
The present invention provides a method and apparatus for producing multiple formations or zones from a single wellbore wherein the wellbore passes through an upper production formation or zone and a lower production formation or zone. A whipstock, having openings therethrough which, in turn, are sealed by dissolvable plugs, is positioned within the wellbore at a point adjacent or above the upper production zone. A lateral wellbore is drilled and completed into the upper zone by using the whipstock. The production from the lower zone is blocked by the whipstock while the lateral wellbore is being completed. As used herein throughout both the specification and claims, when the term xe2x80x9cformationxe2x80x9d is used, it is intended to cover not only distinct subterranean formations but also productive zones within the same formation.
Once the lateral has been completed, the openings in the whipstock are opened by pumping a reagent down the wellbore to dissolve the dissolvable plugs. Once the plugs are dissolved and the openings are open, flow from the lower production zone can now flow through the openings and be produced up through the wellbore along with the flow from the upper production zone.
More specifically, the whipstock of the present invention is xe2x80x9chollowxe2x80x9d in that it is formed with a large cavity within the body thereof. A tubular extension extends from the lower end of the body and provides a fluid inlet for flow from below the whipstock into the cavity. The tubular extension is adapted to be received in a packer/anchor, set within the wellbore, to thereby land and latch the whipstock in its operable position within the main wellbore. When the whipstock is in position, flow from below the whipstock cannot flow past the whipstock.
One or more openings are provided through the tapered face of the whipstock and open into a cavity within the body of the whipstock which, in turn, is comprised of a hardened material, e.g. steel. Each opening is initially closed by a plug of a dissolvable material; preferably a metal (e.g. aluminum, magnesium, etc.) which can be dissolved by an appropriate reagent (e.g. hydrochloric acid). Once a lateral has been completed into an upper formation, a slug of a reagent (e.g. hydrochloric acid) is pumped down the wellbore and into contact with the plugs within the openings in the face of the whipstock. The reagent is allowed to react with the plugs to thereby dissolve the plugs thereby opening the openings to flow. This allows the flow from the lower production formation or zone to flow through the extension, cavity, through the now-open openings in the whipstock, and be produced through the wellbore along with the flow from the upper zone.